1. Field of the Invention
The present invention relates to a method for "data hiding", i.e., a technology to embed information into data, and method for "extracting" the embedded information.
2. Related Art
As a new medium called a DVD is being more and more widely used, it is expected that more and more amount of image and/or audio information is distributed through such medium. Since these digitized information can be easily and perfectly copied by everyone without any deterioration, it is an issue in defining the detailed specification of DVD how to prevent abuse and to protect its copyright. Thus, in order to prevent a third party from illegally copying such information (data), a technology is attracting attention for hiding information into the original data. Such hiding technology is called "data hiding."
Data hiding is a generic term of technology for embedding information into another medium (still images, audio, movies, etc.). It is, unlike encryption, intended not to keep secret information contained in a medium, but to integrate information contained in a medium with information to be embedded.
One of significant features of the data hiding lies in that it is invisible marking. When information is embedded into a medium, it is not added with data bits, but is processed so that it cannot be perceived by human vision (data transformation). Thus, there is no increase in the amount of data due to embedding of additional information. For example, when text or audio information is embedded into an image, only one type of medium could be handled on storage. Another significant feature includes indivisibility of embedded information. Since the data hiding directly embeds the additional information into a data structure of the medium, not in a header or a separate file, the embedded information can be extracted even if the platform or data format of the medium is varied as long as the quality of original data is saved.
With the data hiding technology, it is possible to embed, for example, copy-control information as message data indicating whether or not the data is allowed to be copied. It is possible to arrange a DVD player in such a manner that, when it accesses the media data recorded, it extracts the copy-control information, and, if the information inhibits copying, operates to inhibit the copying operation. Thus, it becomes possible to effectively control copying of data in an ordinary household by providing for the DVD player a mechanism for extracting the embedded information so as to extract and analyze the embedded information simultaneously when accessing the data on the recording medium.
"Nikkei Electronics 4-22, 1996" published on Apr. 22, 1996 discloses the following data hiding technology. FIG. 1 is a schematic diagram for illustrating data hiding and extraction of the background art. In the step for hiding ID information, a frequency spectrum is obtained by frequency converting original media information such as movies, still images, photos and audio with discrete cosine transform or fast Fourier transform. ID information is convoluted in the frequency spectrum of media information through spectrum spreading. The ID information is a random number unique to each purchaser of publication. Normal distribution is used as the algorithm for generating random numbers, with the length being 1000. Then, reverse frequency conversion is performed on the frequency spectrum of the media information added with the ID information to restore the digital publication hidden with the ID information. Although the ID information is hidden therein, the digital publication has little difference from the original so that the purchaser cannot visually recognize such difference.
On the other hand, to extract the ID information, it is necessary to have data on the original publication in addition to data on a digital publication which is assumed to be illegally copied. The obtained data and the original data are frequency converted to obtain their frequency spectra. Difference between these frequency spectra is determined, and compared with the ID information issued by the author. This enables it to identify the purchaser of the publication, so that it can be determined whether or not it is an illegal copy.
One of features of this technology is in use of a frequency space for hiding the data. That is, the media information is converted from a real space to the frequency space, and a frequency component in its frequency spectrum is manipulated on the basis of the ID information. Another feature is that the frequency region being utilized is local. That is, the spectrum spreading is conducted only on the low frequency region in the frequency spectrum of the media information without utilizing high frequency components in it. This is to prevent loss of ID information, which is caused in compressing/decompressing the ID information if it is spread to the high frequency region. In other words, this conventional approach is to locally spectrum spread the ID information in the frequency space of the media information, and to hide the ID information in the entire real space of the media information.
In the conventional approach described above, extraction of embedded information is allowed only when the original data is provided in conducting the extraction. However, information provided for a general-purpose DVD player is only information which is stored on an inserted storage medium, and which is processed for embedding, and additional information which is previously stored, for example, a ROM in the player (probably, standardized information for extraction). As long as the DVD player is a general-purpose device, it is practically impossible to separately provide original data when accessing individual data. Therefore, it is impossible to utilize the conventional approach for which holding of original data is prerequisite for the purpose of controlling copying in an ordinary household.
In addition, it is also necessary as a prerequisite for the utilization for the above purpose that the embedded information can be extracted simultaneously with access, and that the system operation can be controlled in real time. However, the conventional approach is difficult to meet such requirement because it needs enormous cost for frequency conversion or the like which is conducted in extracting the embedded information.